In a digital video network (DVN), digital video signals are communicated among several sites which are geographically separate from one another. Although DVNs have applications in many areas, such as video communications for business, medical, government, correctional, and military institutions, and the invention is applicable to DVNs in general, a primary application of DVNs is in education, and the invention is therefore described in this specification in terms of its application in this area, referred to as a distance learning environment.
In the distance learning environment, the sites are referred to for convenience as teacher (T) and student (S) sites. A typical distance learning session involves a teacher at one site (T) communicating with several student classrooms situated at other sites (S) remote from the teacher's site.
One known form of DVN provides for distance learning sessions involving a teacher site and up to three students sites, referred to as a 3S+T configuration. Each site includes at least one video camera for producing a video signal at the site, at least one video monitor for displaying a received video signal, and a video codec (coder-decoder) for coding and decoding video signals for communication as DS-3 signals. The DVN also includes a controller comprising a DS-3 switch, for switching the DS-3 signals, four video codecs, at least one quad split unit, and four audio mixers. The DS-3 signals provide communications (video, audio, and control signals) in both directions of transmission between each site and the controller.
The quad split unit is a device which is supplied with up to four incoming video signals and produces at an output, selectively, either a video signal which is a composite of the four incoming signals, one incoming picture in each of four quadrants of the outgoing signal picture, or any one of the four incoming video signals. The latter is referred to as a zoomed picture, even though it represents a normal picture, because it occupies the whole of the monitor screen and hence has four times the area of each quadrant of the composite picture.
In use of this DVN, the DS-3 switch in the controller is controlled to set up a desired session among the teacher and three student sites, which may be selected from a possibly much larger number of sites. During the session, the controller (under the control of the teacher) determines what DS-3 signals are sent to, and hence what pictures are displayed on the monitors at, the different sites. For example, at one time the teacher may wish to display a quad split or composite picture at all of the sites; at another time the teacher may wish to display a zoomed picture from any one of the sites at the other sites and a quad split picture at the one site.
It is important for the switching to be effected in a manner which is not disturbing to the students and the teacher, and which ensures that each quad split picture is displayed at each site with a fixed arrangement of pictures from the other sites. For example, it may be required that the quad split picture at each student site always display the teacher's video signal in the top left quadrant, and display the video signals from the other three student sites in fixed positions in the other three quadrants.
Although the known DVN has effected the video switching in an acceptable manner, at least when using suitably fast DS-3 switches, it provides a less satisfactory result for the audio signals. More specifically, it is desirable to provide a continuous audio presence in which each site at all times receives a mix of the audio signals from all of the other sites. Thus each site needs to be supplied with an audio signal which is a mix of all of the audio signals minus the audio signal of the particular site (to avoid feedback problems), commonly referred to as a mix-minus function. In the known DVN, the audio mixers provide this function for sites displaying a quad split picture, but any site displaying a zoomed picture is supplied only with the audio signal related to that zoomed picture. Consequently, there is an undesired audio switching, and loss of continuous audio presence, which accompanies the video switching between quad split and zoomed pictures.
A further disadvantage of the known DVN is its limitation to four sites. An increase to five sites is desirable, considering that the quad split picture enables each site to display pictures from four other sites. Thus it is desirable to provide for a five-site or 4S+T configuration. At the same time, it is required that the DVN easily accommodate lesser configurations, such as 3S+T and 2S+T, for different sessions.
An object of this invention is to provide an improved digital video network which enables the requirements discussed above to be met.